The present invention relates to an electrodynamic sound converter, and more particularly to an electrodynamic sound converter for use in the non-destructive ultrasonic testing of workpieces by means of relative movement of the workpiece and of the testing head, respectively.
Equipment of this general type is known, and during the testing the testing head of the ultrasonic flaw detector is moved along the workpiece surface and/or the workpiece surfaces moved relative to the testing head. In order to obtain exact test results it is necessary that a small air gap be maintained between the testing head and the workpiece, and that this air gap be as constant as possible. Problems are encountered during this type of testing if the workpiece surface is uneven. It is known to support the entire testing head on rollers which roll along the workpiece surface and this is a viable solution so long as the relative movement between testing head and workpiece is comparatively slow, because the unevenness of the workpiece surface can be compensated with this arrangement and the air gap can be maintained constant or substantially so. The same is not true, however, if the workpiece and the testing head are displaced at high relative speeds, or if the workpiece surface has unevennesses which follow one another very closely. The problem is that in either of those possibilities the rollers supporting the testing head on the workpiece surface in effect jump from high point to high point of the workpiece surface and the overall testing head--which has a relatively high inertia due to its mass--requires too much time to accommodate itself sufficiently to the depressions so as to maintain a uniform air gap.
Another problem which is independent of the speed of the relative movement is encountered in the prior art if the workpiece surface has closely adjacent unevennesses (i.e. depressions and projections) and if the distance of the spacing members (i.e. rolls or the like) from one another as seen in the direction of relative movement is of the same order of magnitude as the length of the unevennesses. Here, again, the desired constancy of the air gap between the testing head and the workpiece surface cannot be maintained.